Internal tide generation by seamounts, ridges, and islands

A numerical investigation is made into the generation of semidiurnal internal tides by tidal flow over steep topographic features in the deep ocean. A fully three‐dimensional, free surface, nonlinear, hydrostatic model (the Princeton Ocean Model) is used with real stratification, representative tidal forcing, and Gaussian‐shaped seamounts, ridges, and islands for the topography. The efficiencies of the different topographies in extracting energy from the barotropic tide and in generating an internal tide are considered. Each topography produces an internal tide characterized by signals propagating away from the feature as beams that follow internal wave characteristic paths. The strength of the signals, however, varies markedly for different topographies. This is largely attributed to the way in which the barotropic tide interacts with the three‐dimensional topography. Internal wave generation requires a significant vertical displacement of stratified water by the barotropic tide as it flows over topography. It is found that for symmetric seamounts and islands the barotropic flow tends to go around the feature, producing only a weak internal tide. When elongated into a ridge, barotropic flow is forced across isobaths, generating an energetic internal tide. Changing the horizontal aspect ratio of the topography from 1:1 (a seamount) to 3:1 (a ridge) increases the resulting baroclinic energy flux by nearly an order of magnitude. The slope of the topography relative to that of the internal wave characteristics is also shown to be important with strongest generation occurring at regions of critical or supercritical topographic slope.